Acoustic device



A .22, 1944. E, E OTT 2,356,262

' ACOUSTIC DEVICE Filed Aug. 5, 1940 Sheets-Sheet 1 F/GLJ 2a: 27 I V 34 3/ 40 INVENTOR EE. MOTT ATTORNEY POLE PIECE AREA-"SQUARE INCHES Aug. 22, 1944.

E. E. MOTT 2,356,262

ACOUSTIC DEVICE Filed Aug. 5, 1940 3 Sheets-Sheet '2 DIAPHRAGM 0IAMErER= 1.5"

POLE P/EC'S- 4574 PERMALLOY DISTANCE .sErwEE/v POLE PIECES-3% SEPA .94 7'10 005" D/APHRAGM THICKNESS /NCHE$ FIG. F/G. /2

if I I 5 M, R;

FIG/3 RESPONSE "DE C IBEL .S ABOVE THRESHOLD 5 l l l FREQUENCY-K/LOCYCLEE PER SEC.

lNl/ENTOR E E. MOT T BY 'VhwmaM A T TORNE V Aug. 22, 1944. E, 'E. MOTT ACOUSTIC DEVICE 5 Sheets-Sheet 3 Filed Aug. 5, 1940 FIG. /0

FREQUENCY INVENTOR E E. M0 T T B) A T TORNEY PltcntcdAug. 22, 1944 ACOUSTIC DEVICE Edward E. Mott, Upper Montclair, N. 1., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 3, 1940, Serial No.. 350,741

9 Claims.

This application is a continuation of the application Serial No. 93,792, filed August 1, 1936, of

-Louis A. Morrison and Edward E. Mott, which resulted in Patent 2,220,942, granted Nov. 12, 1940.

This invention relates to acoustic devices, and, more particularly, to telephone receivers of the magnetic armature or diaphragm type.

Objects of the invention are to increase the sensitivity of such devices; to extend the frequency range of their efllcient operation; and to improve the uniformity of their responses in the operating range.

These objects are accomplished in part by the provision of an improved magnetic circuit in which the structural proportions of the elements and the magnetic properties of the materials are coordinated, in a manner hereinafter described, to provide an optimum force factor. In part, they are accomplished by the use of arrangements which not only provide an adequate degree of damping, but also serv to diminish substantially the effectof the inertia of the diaphragm.

An important feature of the invention lies in the mutual proportioning of the cross-sectional area of the pole-pieces of the magnet system, the thickness of the diaphragm or armature, and the magnitude of the polarizing flux whereby the ratio of the force factor to the effective moving mass is substantially the maximum obtainable for the materials of the magnetic circuit.

Another feature is the provision of improved magnetic alloys for the several parts of the magnetic circuit, the materialof each part being particularly adapted to the function thereof.

A further feature resides in a relatively thick and rigid diaphragm having a high magnetic efficiency in combination with damping means proportioned to diminish substantially the effec tive mass.-

An additional feature includes a freely supported diaphragm, whereby variable mechanical stresses due to temperature changes and the like are avoided and stability of the operating characteristic is secured together with structural arrangements for securing and maintaining accurate relative positioning of the diaphragm and the pole-pieces.

Other and further features will be evident from the description which follows hereinafter.

A more complete understanding of the invention will be obtained from the following detailed description, taken in conjunction with the appended drawings, wherein:

Fig. 1 is a top plan view of a telephone receiver or receiver unit embodying the invention;

Fig. 2 is an enlarged rear plan view of the device of Fig. 1;

Fig. 3 is a sectional view of the device of Fig. 1, taken along the line 3-! of Fig. 2; r

Fig. 4 is another sectional view of the device of Fig. 1, taken along the line 4-4 of Fig. 2;

Fig. 5 shows the device of Fig. 1 associated with the receiver end of a hand telephone or handset;

Fig. 6 shows in perspective and partly broken away the pole-piece and magnet assembly of the device of Fig. 1;

Fig. 7 discloses, embodied in a hand telephone, a modification of the invention;

Fig. 8 is an electrical circuit analogy of the electrical and acoustical elements embodied in a receiver in'accordance with this invention;

Fig. 9 shows a set of contour curves illustrating the variation in figure 01' merit of telephone receivers with variation in the pole face area and the diaphragm thickness thereof for a magnetic circuit of a given air-gap length and given magnetic materials;

Fig. 10 shows electrical and acoustical characteristic curves for a telephone receiver in accordance with the invention;

Fig. 11 is a schematic of the damping elements of the device of Fig. 7;

Fig. 12 is a schematic equivalent of Fig. 11;

Fig. 13 is an amplitude-frequency response characteristic of a telephone receiver in accordance with this invention, the dotted line indicatin the effect of air leakage between the receiver and the users ear.

The device illustrated in Figs. 1 to 6 is a telephone receiver comprising a unitary receiver assembly 20, adapted for mounting in the receiver end of a handset or other type of receiver case and to be held in position by the receiver cap. The assembly of the device in a handset is illustrated in Fig. 5 in which 2| is a portion of a handset frame, the receiver end of which is provided with a recess 22 surrounded by a raised annular rim 23 on which the assembly rests. Receiver cap 24 threaded to the annular rim 23 clamps the device in position.

The central feature of the unit 20 is a rigid annular foundation frame 25 to which the other' elements of the assembly are attached. This frame is preferably of non-magnetic metal, such as aluminum or zinc alloy, and may be of die cast construction.

Bosses 26, projecting from the back of frame 25, support a unitary magnet and pole-piece system, the detailed construction of which is shown frame 25 by rivets 42.

iacent and are welded at their ends to the flanges of the pole-pieces, the complete assembly having the form indicated. The unitary structure is mounted on frame 25 with the cross portions of the pole-pieces resting on projecting bosses H and secured thereto by screws 29. Terminal plate III of insulating material extending across the tic part. It is similar in shape to the mine 2! of the preceding example, but includes the damping plate back of the diaphragm as an integral rear of the assembly is also secured in position by screws 28. The ends, of the pole-pieces pass between the magnets and project into the central aperture of the foundation frame. Speech current coils 3| are mounted on the pole-pieces and have their ends brought out to concentric contact elements 32 and 33 mounted on plate 30.

The front face of the foundation frame 25 is provided with an annular ridge ll surrounding a central recessed portion. A flat circular diaphragm 35 of magnetic material is supported at its periphery by the ridge, and is held in position by the attraction of the pole-pieces. The diaphragm is not mechanically clamped and is therefore free to expand and contract in response to temperature changes without being subjected to mechanical stresses. The form of the supporting frame 25 and the method of supporting the magnetic elements therefrom facilitate accurate machining and assembly of the parts and ensure the establishment and maintenance of a precise separation between the diaphragm and the faces of the pole-pieces.

A damping plate 38 of insulating material, provided with suitable close fitting apertures through which pole-pieces 28 project, is inserted in the central recessed portion of frame 25 to which it may be riveted, cemented, bonded or otherwise secured. The damping effect is obtained by the use of a plurality of very narrow air leakage paths leading through plate 36 from the air chamber 38 between the plate and the diaphragm. In the present structure, these are provided by an aperture 40 in the damping plate over which is laid one or more strips of silk fabric 50. Other known arrangements for this purpose may also be used, such as any fibrous or porous material through which th air may flow against frictional forces. The front of the diaphragm is enclosed by a recessed cover plate ll of insulating material, which is secured to The recessed portion of the cover plate is large enough in diameter to clear the diaphragm and is deep enough to provide an air chamber 43 of proper dimensions in front thereof. a. group of small, centrally located apertures 44 provide sound wave outlets from air chamber 43.

A typical magnet structure would comprise parts approximately of the following compositions and dimensions: permanent magnets of 35 per cent cobalt steel, 1.25 inches long, and onesixteenth of a square inch in cross-sectional area; pole-pieces of a nickel-iron alloy in the approximate proportions 45:55, each having a cross-sectional area of .031 square inch; a diaphragm of cobalt iron-vanadium alloy in the approximate proportions 49z49z2, its thickness being .011 inch, its diameter 1.46 inches and its normal air-gap from the pole faces substantially six-thousandths of an inch.

The receiver illustrated in Fig. 7 is, in general, similar to that of the preceding figures, but differs therefrom in certain features of the mechanical structure and in the arrangement of the acoustic damping elements. The foundation frame 45 is of insulating material instead of element with apertures molded to receive the magnet pole-pieces. The magnet system and the diaphragm are of the same construction, and are supported in the manner already described. The complete assembly unit is adapted for mounting in the same way as that of Figs. 1 to 6, and may be interchangeable therewith. In the figure, it is shown mounted in a handset telephone.

For the purpose of damping, an aperture ll in the front of frame 45 communicates with a chamber 41 formed in the rear of the frame. A disc 48 positioned across the rear of aperture 48 and provided with a plurality of fine holes or slots provides the desired acoustic resistance. Chamber I1 is closed by a back plate 49, cemented in place, thereby limiting the motion of the air through the acousticresistance and modifying the action thereof. In an experimental model of this receiver, the magnetic structure-was the same as that of the previous example except that a diaphragm of slightly larger diameter, 1.565 inches, was used.

The invention resides, in part, in the coordination of the structural proportions of the mechani-' cal elements with the magnetic characteristics of the materials of the system and with the acoustic elements of the system. The mechanical structures illustrated in Figs. 1 to 7 are of importance in that they embody features whereby the principles of the coordination may be applied with a high degree of accuracy, and whereby the stability of the improved operating characteristics is ensured. The manner in which the proportions of the system are coordinated will be more readily understood and appreciated from the following considerations.

The function of a telephone receiver of the type described is to produce a pressure variation in the cavity of the ear to which it is applied, which follows as faithfully as possible the variations of the speech current applied to the device. Since the ear cavity forms a substantially closed air space directly coupled to the diaphragm of the receiver, the pressure variation will be dependent on the amplitude of the diaphragm motion rather than on its velocity. While the air space enclosed by the application of a receiver to the earls, in general, not completely closed, we have found that the effect of the air leakage paths is large only at quite low frequencies, and is barely noticeable at frequencies above 200 or 300 cycles per second. It is assumed, of course, that the instrument is held fairly firmly against the ear.

For the purpose of examining the response characteristic of any given receiver, we have found that it is permissible to replace the ear cavity by a rigid-walled chamber of six cubic centimeters volume coupled directly to the outlet aperture in I the receiver cap and having no outlet to the exmetal, and may consist of a molded pheno p asternal air. The pressure variation measured in this cavity is a reliable measure of the sensitivity of the receiver and furnishes an accurate basis for the comparison of difl'erent designs. The use of rigid walls in the chamber is justified by the fact that the energy absorbed by the walls of the ear cavity is negligibly small compared with the energy dissipated elsewhere in the receiversystem. The volume six cubic centimeters, has been found by many measurements to be a good approximation to the normal human ear, and is, therefore, representative of the conditions under which the receiver will most frequently operate.

tion source of voltage I and internal ruistan'ce R. i

The receiver winding impedance, measured. with the diaphragm held fast, is representedby resistances n and r: and inductances; L1 and L1. Resistance n is the direct current resistance of the winding. inductance L1 is the inductance of that part of the winding which is substantially uncoupled to the magnetic circuit, inductance L1 is the inductance 'ofthe coupled, or effective, part of the winding. and resistance 1': represents the eddy current loss in the magnetic elements. The electrical part of the circuit, is coupled to the mechanical-acoustical part by virtue of the force factor which is indicated schematically by the element G. The elements of the mechanicalacoustical part of the system are indicated in the diagram as follows: Mo denotes the effective mass of the diaphragm, So its effective flexural stiffness, and Re the mechanical resistance due to hysteresis. Thcstiifness of the air chamber 38 back of the diaphragm is represented by S1 and the acoustic impedance of the leakage path through damping element 40 by resistance R1 and mass M1. in front of the diaphragm, R3 and M: are respectively the'resistance and mass of the combined outlet apertures M, and S: is the stillness of the enclosed ear cavity which, for the reasons indicated, is taken as that of an air volume of six cubic centimeters connected to apertures. Resistances R0 and R: are sufllciently small in comparison with the other resistances of the system so that they do not-materially affect the response. At very low frequencies, where the reactances of the mass and inductance elements are negligibly small, it may be shown that the amplitude of the displacement of the diaphragm is given by wherein a denotes the amplitude, G the magnitude of the force factor,. and I the amplitude of the current in the electrical circuit, all values being in c. g. s. units. Si maybe omitted because it is negligibly small. If the masses of the system could be eliminated, no damping would be necessary, sinceQin the absence of mechanical resiststantially constant, but at higher frequencies it tends to fall off very rapidly. The resonance frequency of the diaphragm is, therefore, a measure of the frequency range through which uniform response can be maintained. An extension of the frequency range above the diaphragm resonance is obtained by proportioning the outlet apertures 44, which couple the front of the diaphragm with S: denotes the stiffness of the air chamber the ear cavity, so that the mass of air therein resonates with the stiffness of the ear cavity and the front air chamber 0 at a higher frequency. This resonant-system corresponds to the loop Ba, Ba, M1. Rs, in Fig. 8. However, to maintain uniformity of the response over the extended range, the above noted ear cavity resonance frequency must bear a substantially fixed proportion to the.

resonance frequency of the diaphragm, which proportion I have found to be substantially equal to 1:3. The utility of the diaphragm resonance frequency as a measure of the operating range is, therefore, not affected by modifications of the acoustic system.

The diaphragm resonance frequency is given by the relationship.

the left-hand side of which represents the product of the displacement per unit current multiplied by the square of the frequency range, and

the right-hand side of which is a constant for a given structure. Since the performance of a receiver is gauged by the combination of the two factors, sensitivity'and frequency range, it will be seen from Equation 3 that a high level of performance depends not on the magnitude of the force factor alone, but on the ratio of this quantity to the eifective mass of the diaphragm.

In Equations 1 and 3, the response is indicated by the amplitude of the diaphragm displacement More correctly, it should be represented by the pressure increment in the ear cavity represented by the stiffness 8:. At low frequencies, and hence at all frequencies in the response range of a properly damped device,,the increment of pressure due to the diaphragm displacemehtis' given by in which the sensitivity of the device appears as the pressure increment in the ear cavity per unit of current in the receiver windings. The quantities P and 1 are physical constants and. since the volume V: will generally be small compared with Va, the sum of the two volumes may be taken as substantially equal to V3, which may also be considered to be a physical constant. The ratio P'y/(Vz-i-Vs) is, therefore, a constant which is independent of the receiver design.-

It may, further, be shown that the force factor G, which is defined as the ratio of the mechanical force produced to the magnitude of the current producing it, may be expressed in terms of the magnetic fiux and the number of turns in the receiver winding by the equation,

where n is the number of turns and Go, or

. l ?l 491 V (In,)*v,+ v. M. I m in which the sensitivity is expressed as the more basic ratio of the acoustic pressure to the-ampere turns of the winding. The factor Ge appearing on the right-hand side'is likewise a basic parameter of the magnetic structure since it depends wholly on the geometry of the structure and the magnetic properties of the materials. The expression,

may, therefore, be regarded as a fundamental figure of merit by means of which the performances of different types of telephone receivers may be compared. In the case of receivers..suc h as those of the invention, in which magnetic diaphragms of uniform thickness are used, the ratio of the effective area to the effective mass is substantially inversely proportional to the diaphragm thickness, the densities of the various ferro-magnetic materials being approximately alike. The figure of merit is therefore D D rtional to the ratio of the force factor to the diaphragm thickness.

To secure a high figure of merit, it is desirable that the diaphragm have a large effective area and a small effective mass, and that the flux traversing the magnetic circuit vary rapidly with the diaphragm displacement. The value of the figure of merit is dependent on the length of the air-gap, the crosssectional area of the polepieces and the thickness of the diaphragm. all of which may be varied independently. As the airgap is decreased, the figure of merit increases continuously, but practical considerations of manufacture and of stability-of operation limit the minimum length of air-gap. and hence, also, the improvement in the figure of merit that can be obtained in this way.

From an extensive series of measurments of the force factor in structures employing electromagnets as the source of polarizing fiux I have found that, for a given set of structural dimensions, air-gap, pole face area and diaphragm thickness, the figure of merit exhibits a maximum for a particular value of the polar magnetizing force. Further, when the diaphragm thickness is varied independently, the magnetizing force being adjusted in each case to the optimum value, a maximum of the figure of merit appears for a particular diaphragm thickness and, likewise, when the pole face area is varied a maximum is developed for a particular area. I have also found that the figure of merit reaches a grand 'maximum for a particular combination of these dimensions. This represents an absolute maximum of the figure obtainable with a given air-gap and given materials in the pole-pieces and diaphragm. To achieve the maximum there maximum with respect to variations of these quantities, but to obtain the absolute maximum it is necessary that the polarizing fiux also have a value for optimum effect.

In the experimental work referred to, the rate of change of the magnetic flux was measured directly by giving the diaphragm a sudden minute displacement of accurately measured magnitude and observing the resulting deflection of a calibrated ballistic galvanometer connected to the receiver coil terminals. This method proved sensitive and accurate and sufficiently rapid to permit a large number of conditions to be examined in a very short time. In the tests, the pole face area, the diaphragm thickness and the length of air-gap were systematically varied and a wide range of magnetic materials was investigated. The variation of the figure of merit found in one series of tests is illustrated by the closed curves of Fig. 9. Each curve or contour line corresponds to a fixed value of the figure of merit and the coordinates represent the various combinations of pole area and diaphragm thickness which provide this value. The intersections of the contour lines with horizontal lines parallelto the thickness axis give successive values of the figure of merit for different diaphragm thicknesses, and the intersections with vertical lines give the values for different pole face areas.

In the series of tests resulting in the curves of Fig.- 9, the magnetic structure used was generally similar in configuration and in materials of construction to that of the receiver shown in Figs. 1 to 6, -except that a slightly smaller air-gap of five thousandths of an inch was used. The maximum value of the figure of merit was somewhat greater than 25x10 in c. g. 5. units and was obtained with a pole face area of approximately .030 square inch, and a diaphragm thickness of approximately eleven thousandths of an inch.

feet on the optimum dimensional relationship and resulted only in a slight diminution of the maximum figure of merit.

In order that the optimum conditions might be realized in a corresponding structure employing a permanent magnet, the size and strength of the permanent magnet were so chosen as to produce the same polarizing fiux in the pole-pieces and air-gap as was produced by the optimum magnetizing current in the experimental electromagnet system. This was satisfactorily accomplished by the use of a pair of 35 per cent cobaltsteel magnets, 1.25 inches long and one-sixteenth of a square inch in cross-sectional area, magnetized to a stable condition in accordance with standard practices. Further tests were made- The physical conditions that give rise to the V optimum dimensional relationships are of a complex character and do not lend themselves readi- 1y to analysis. However, the existence of the grand maximum of the figure of merit has been thoroughly established by the tests referred to above. Moreover, the tests show that the grand maximum has the character of an absolute maximum which, for given magnetic materials and for a given air-gap cannot be exceeded, except, possibly, to the extent of second order variations, by any changes in the configuration or proportions of the structure or of the valu of the polarizing fiux. With different magnetic materials, other optimum dimensional relationships than those indicated by Fig. 9 may be found to hold. The tests referred to above have shown that, in general, the optimum dimensional relationships involve a diaphragm thickness much Kreater than has heretofore been used. For example, when diaphragms of ordinary magnetic iron are used, the optimum thickness is found to to be approximately fourteen-thousandths of an inch or greater. The optimum dimensions and the optimum polarizing flux for any particular combination of materials may be determined by following the experimental procedure outlined above.

In the present receiver the diaphragm material is characterized by a permeability which retains a high value at high flux densities. It is thought that this characteristic may, at least in part, account for the fact that the optimum diaphragm thickness is less than for other magnetic materials.

From the general formula for the attraction lowed in making such redesign will be evident between the faces of an air-gap in a magnetic circuit, it can be shown that the force factor G is expressed approximately in terms of the magnetic fluxes by the equation,

da 41r d1; d.

where B0 is the polarizing flux density in the. airgap and (Lt/dz is the rate of variation of the flux with current in the receiver winding. The latter quantity is inversely proportional to the reluctance of the magnetic path traversed by the alternating'fiux produced by oscillating currents in the receiver windings. It follows, then, that the figure of merit expressed by Formula 8 is proportional to the ratio of the polarizing flux density to the alternating current reluctance of the magnetic circuit and inversely proportional to the diaphragm thickness. These quantities influence each other mutually in a highly complex manner which prevents a mathematical analysis of the conditions existing at the optimum relationship. The experimental attack outlined above, therefore, represents the best available method for obtaining accurate data.

supported freelyat its-periphery without danger of being pulled into contact with the pole faces by the attraction due to the polarizing flux. The rigidity is also sufficient to ensure the maintenance of a fixed normal air-gap thereby stabilizing the operating characteristics of the system. The absence of edge clamping results in a simpler mode of ilexure of the diaphragm under the superimposed force of the speech currents which considerably increases the effective area and the total volume of the air displaced.

While the pole-pieces have been described hereinabove as L-shaped with side flanges, it is to be understood that considerable variation is permissible in structure. Thus, the side fianges may from this specification.

A further improvement in the over-all performance of the receivers of the invention is obtained by the use of damping arrangements which not only provide the energy dissipation necessary for uniform response, but at the same time substantially diminish the effective mass of the diaphragm. This result is achieved by'tlie proper coordination of the acoustic and electrical damping whereby the negative reactances introduced I by the separate means are'of a complementary character,. their sum increasing substantially linearly with frequency throughout the greater portion of the operating range.

At the grand maximum, the variation of the figure of merit with the pole face and diaphragm dimensions is not very rapid and a reasonable latitude in the dimensions of these parts may be permitted without noticeable sacrifice of the sensitivity or frequency range. For the magnetic similar type.

The relatively great diaphragm thickness for maximum force factor to mass ratio provides suflicient rigidity so that the diaphragm may be R1, mass M1 and stiffness Si.

Referring to Fig. 8, the acoustic damping elements of the system are represented by resistance If this combination is proportioned so that R1 is approximately equal to the square root of the product M181, its total effective reactance will vary with frequency in the general manner indicated by curve A of Fig. 10. The characteristic features of the variation are a low value of reactance throughout a wide range at low frequencies and a maximum negative value at a relatively high frequency close to the resonance frequency of'mass M1 with stiffness Sn.

Another component of negative reactance is introduced by virtue of the coupling to the electrical circuit through the force factor. The impedance introduced into the mechanical-acoustical portion of the circuit in this way is equal to which is the impedance of a network of inverse character to the electrical impedance R, n, Ll, rz, L2. The reactance portion of the introduced impedance has a characteristic'variation of the type shown by curv B of Fig. 10 and is negative in sign. 'By proper choice of the electrical coefficients, curve B may be made substantially complementary tov curve A, giving a total efiective reactance of the type indicated by curve C. This curve represents a negative reactance which increases substantially linearly with frequency up to or beyond the resonance determined by M1 and S1. The frequency variation of the reactance is of the same character as that of the reactance of a physical mass, but the sign of the reactance is reversed. The damping elements therefore may be considered as contributing a negative mass which is nearly constant over the operating range and which, being in series with the diaphragm mass, diminishes the total effective mass of the system. Physically a negative mass would have the property of producing an inertia reaction which acts in the direction of the acceleration instead of in opposition thereto, and may be considered to-be present in any system where such a condition exists.

Heretofore in, receivers of the magnetic diaphragm type, the force factors have been so low that the requisite values of the transferred impedance to provide the desired characteristics could not be obtained-without a very great sacrifice of efilciency. Because of the large force factors obtaining in the receivers of the invention, the desired negative mass effect is obtained without substantial loss of eillciency.

In the modified form of the invention shown in Fig. 7, the damping elements take the schematic form shown in Fig. 11, which differs from that of Fig. 8 by the addition of a stiffness Sm,

directly in eries with the mass and resistance.

This added tiifness is that of the enclosed chamber 41 behind the perforated plate ll. The effect of the closed chamber is to diminish the volume and velocity of the air leakage through the perforations, and, hence, to reduce the rate of energy dissipation. To compensate this reduction, the resistance of the damping element must be increased when the closed chamber is used. Another effect is to add a stiflness restraint to the diaphragm equal to the stillness of the total enclosed space behind the diaphragm. In Fig. 11, S01 is the stiffness of the enclosed chamber 41, M1 and R1 are, respectively, the mass and resistance of the apertures in damping plate ll, and S1 is the stiifness of air chamber between the diaphragm and frame 40. C

By means of a transformation theorem described by 0. J. Zobel, Bell System Technical Journal, January 1923, Theory and design of uniform and composite electric wave filters, Appendix III, Transformations A and B, it may be shown that this modified damping can, by suitably proportioning the elements, be made the full equivalent of the combination Mi, R1, Si, of Fig. 8, together with a stiffness added in series externally to the group. The equivalent schematic is shown in Fig. 12 in which 84 represents the added stiffness.

The curves of Fig. 10 represent the characteristics of an experimental device of the type shown in Fig. 7 for which the mechanical and acoustic constants had the following values:

The corresponding constants of the equivalent damping system, Fig. 12, are:

R1= 2330 mechanical ohms M1=1241 gram 81 31.8)(10 dynes per cm. 54:17.4 10 dynes per cm.

The mass reactance of the diaphragm is represented by straight line D and the effective mass by curve E, the difference between the two being the negative mass reactance indicated by curve.

C. Dotted straight line 1'' represents the average value of the effective mass reactance. The reduction of the diaphragm mass in this instance is substantially 60 per cent. The total reactance of the system which is the resultant of the effecv tive mass reactance and the combined reactancog of the diaphragm stiffness Bo, rear chamber stiff; ness St, and front chamber stiffness 8s and 8:, is represented by curve G. The total reactsnce is zero at 2150 C. P. 5., corresponding to the effect tive resonance frequency of the diaphragm.

From the constants given above, the various significant resonance frequencies of the system may be determined. The values are as follows: The diaphragm by itself, that is, in the absence of acoustic stiifness restraints has a resonance frequency of 1000 C. P. 8. With the addition of the acoustic restraints represented by stiffness 84 and the parallel connection of Sr and 8c, the diaphragm resonance is increased to 1400 C. P. 8. The acoustic damping system comprising resistance Rx, stiffness B1 and mass Mi is proportioned so that M1 and S1 resonate at about 1850 C. P. 8. The load system represented by ear cavity simness 8:, mass Ma, and front chamber stiffness B: is resonant at about 2700 C. P. 8. Further, as shown by curve G of m. 10. the final effective resonance of the diaphragm occurs at a frequency in the neighborhood of about 2150 C. P. 8., the increase from 1400 C. P. 5. representing the effect of the mass reduction due to the combined electrical and acoustical damping.

To secure uniformity of response I have found it desirable to maintain certain relationships among these resonance frequencies. Preferably, the three resonance frequencies of the diaphragm and the ear cavity resonance should form approximately a geometric series with a ratio of the terms of about 1.4:1 or 1.5:1. Under this condition, the extension of the response range above the resonance frequency of the diaphragm itself is contributed in about equalparts by the acoustic stillness, the reduction of mass due to damping, and the resonance of the ear cavity. The frequencies 1000, 1400, 2150 and 2700 C. P. S.mentioned, correspond quite clouly to the series, 100.0:l400z1960z2'l50. In addition, the resonance frequency of the rear damping system should be approximately equal to or greater than the final effective resonance of the diaphragm, in order that the mass reduction may be effectiv ova substantially the whole operating frequency range. In the foregoing example, the values are 1850 C. P. 8. and about 2150 C. P. 8., respectively, which is satisfactorily close. I

Inthedevice accordingtoFigs. 1to0,theabsence of the acoustic stiffness 84 tends to reduce the effective resonance frequency of the diaphragm. However, since the mechanical stiffness of the diaphragm varies as the cube of the thickness, the reduction may be compensated by a slight increase in the diaphragm thickness, without noticeable sacrifice of the m m of merit.

nance appear asslight undulations at 2150' C. P. S. and 2800 C. P. 8. At low frequencies. the. full line curve represents the response in a completely closed ear cavity corresponding to the reference conditions previously described. The

dotted curve shows the effect of normal air leakage between the receiver cap and the car. At a frequency of 3000. P. S. the loss of sensitivity is decibels and increases rapidlywith decreasing frequency. Since frequencies below 300 C. P. S. are of relatively small importance in speech transmission, the limitation of the response range at the lower range is not of consequence. At higher frequencies the effect of normal air leakage is negligible.

As heretofore stated, a single bar magnet such as 21, may be used instead of two of such magnets as shown in Fig. 6. In case a single magnet is employed, it may be made of an alloy comprising iron, cobalt and molybdenum in the approximate proportions 72:12:16 and have a cross-sectional area of about one-sixteenth of a square inch and a length of about 1.25 inches.

Reference is made to Patent 2,231,084 granted Feb. 11, 1941, on an application Serial No. 161,936, filed September 1, 1937, a division of the application Serial No. 93,792, filed August 1, 1936, wherein certain features of the devices described hereinabove are claimed.

What is claimed is:

1. In a sound translating device, a moving I system comprising a vibratory diaphragm and having a predetermined response characteristic over an operating frequency range when arranged for substantially free vibration, means defining a first chamber adjacent the inner side of said diaphragm having a passage communicating therewith, the dimensions of said passage being so proportioned that said first chamber acts substantially entirely as a damping chamber effective substantially to decrease the response of said moving system at frequencies within a band of frequencies extending toward the low frequency end of said range, and means defining a second chamber adjacent the outer side of said diaphragm having a passage communicating therewith, the dimensions of said last-mentioned passage being so proportioned that said second chamber acts substantially entirely as a resonating chamber effective substantiallyto increase the response of said moving system at frequencies within a band of frequencies extending toward the high frequency and of said range.

2. Ina sound translating device, a housing, a diaphragm mounted within said housing, said diaphragm having a predetermined response characteristic over an operating frequency range when mounted within said housing for substantially free vibration, said diaphragm having a A characteristic of said diaphragm and substantially to decrease the response of said diaphragm at frequencies within a band of frequencies extending from a frequency of the order of 1500 cycles per second toward the low frequency end of said range. means defining a second chamber adjacent the outer side of said diaphragm having a passage communicating therewith, the dimensions of said last-mentioned passage being so proportioned that said second chamber acts substantially entirely as a resonating chamber eiIective substantially to increase the response of said diaphragm at frequencies within a band of frequencies extending from a frequency of the order of 2000 cycles per second toward the high freat all frequencies in the range to be reproduced.

4. In combination in a telephone receiver, a diaphragm having a substantial mass effect over the frequency range to be reproduced, and means introducing a substantially uniform negative mass effect throughout said range to reduce the effective mass of the diaphragm in the order of 50 per cent.

5. A telephone receiver comprising a diaphragm having a natural period in the frequency range to be, reproduced, and acoustic means associated therewith, said means possessing the property of introducing resistance to flatten out the resonance peak of the diaphragm vibrations and negative mass reactance to reduce the effect of the mass reactance of the diaphragm substantially uniformly throughout a range of at least 1000 cycles above said natural period.

6. A telephone receiver comprising a frame member containing an air chamber and an aperture connecting with said air chamber, acoustic damping means across said aperture, a magnetic diaphragm on said support and forming a shallow air chamber with said frame, said first and second chambers being completely closed except for connection to each other through said aperture, and means for exerting a constant and a superimposed alternating magnetic force on said diaphragm, said diaphragm being maintained in position by magnetic forces only.

natural frequency of vibration such that said response characteristic peaks at frequencies within the low frequency end of said range, means defining a first chamber adjacent the inner side of said diaphragm having a passage communicating therewith, the dimensions of said passage being so proportioned that said first chamber acts substantially entirely as a clamping chamber effective to remove said peak from the response 7. A telephone receiver comprising a diaphragm having a positive mass reactance increasing substantially linearly with frequency, and means coupled to said diaphragm defining a negative reactance increasing substantially linearly with frequency.

8. A sound translating device comprising a diaphragm having a natural period of vibration in the frequency ran e to be translated, electromagnetic signal translating means cooperatively associated with said diaphragm, means defining an acoustic damping system adjacentone surface of said diaphragm effective to flatten out the resonance peak of the diaphragm vibrations, said system comprising resistance, mass and stiffness, the resistance being of the order of the square root of the product of the mass and stiffness, and

means defining an acoustic resonating system adjacent the other surface of said diaphragm effective to increase the response of said diaphragm throughout a band of high frequencies in said range, said resonating system having substantially negligible resistance.

9. A sound translating device comprising a dia.

phrasm having a natural period of vibration of the order of 1,000 cycles per second, electromasassumes .i'ective resistance of said network is reater than the eii'ectivc reactanoe of said network throw!!- out a ranse of frequencies extending from below i,000 .cy cles per second to of the order or 2,000 cycles per second, and-means denning a second acoustic network adiacent the other surface of said diaphrasm. said second network having substantially nezlisibie resistance and beinl resonant attheorderoflflw cyelesper'seeond.

mwann 1 non. 

